8/13/2019 DTI Spec Sheet

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DTI’s

A bolted joint obtains its superior characteristics throughproper clamping force on the gasket contact surfaces. Theclamping force, or bolt load is caused by correctly tensionedbolts. If the bolts have not reached required tension, there

is insufficient clamping force and the joint is not up tospecification. If excessive clamping force is used, the bolt,gasket, and/or the flange may be damaged. In either case, aleak is the probable result. Therefore, it is imperative that theproper clamping force is achieved. Direct Tension Indicatorsprovide the means to measure bolt tension (bolt load).

  The bolt is tightened to a specified DTI gap

which has been achieved directly by clamping force, ortension. This means you are measuring the ac tual outcome of

your efforts instead of work input. Accuracy is not affected bybolt grip length.

  Direct Tension Indicators are manufacturedin small lots, and each lot is tested for consistency. A testreport is kept and its lot number is marked on each DTI. Inuse, if the DTI’s are compressed to the gap specified, each boltis proved to be tensioned over the minimum, and under themaximum load.

  Inspection is cost effective with DirectTension Indicators because they are in use as long as thefastener is, do not required any special training to use or to

inspect, and they help prevent rework.

   The DTI can be used under the bolt head or atthe nut end with a hardened washer, and it will take up to a1:20 bevel (see figure 4).

  DTI’s are easyto install with standard tools – proper clamping force isunmistakable.

  The Direct Tension Indicatorprovides immediate visual proof that the bolt has beencorrectly tensioned. All you need is a feeler gauge.

  Direct Tension Indicators are madeto fit bolts manufactured to ASTM A193-B7 and B16, as wellas equivalent metric sizes and specifications. DTI’s can bemanufactured and tested for use with customer specifiedbolt materials.

Direct Tension Indicators measure clamping forces(bolt load)The Direct Tension Indicator (DTI) is a specially hardened washer with protrusions on one face (see figure 1). The DTI is placed under the bolt head or

nut, and the protrusions create a gap. As the bolt is tensioned, the clampingforce flattens the protrusions, reducing the gap (see figure 2).

StationaryNuts

Gap Gap

HardenedSteel Washers

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DTI’s

Correct bolt tension is evaluated by simply observing the remaininggap. A “no-go” feeler gauge is used to insure that minimum specifiedbolt tension is achieved. A “go” feeler gauge is used to insure thatmaximum specified bolt tension is not exceeded. DTI’s stay on the

 job, providing permanent visual and measurable proof that the boltis correctly tensioned to specification. Gap corresponds to bolt loadverified by a test certificate traceable to NIST.

A 1 1/4” diameter B7 bolt loaded to 50% of minimum yield will have aclamping force (tension) of 52,500 pounds. A DTI for this 1 1/4” B7 boltwill have a remaining gap of .039” at 52,500 pounds of load. A “no-go” feeler gauge .039” thick will indicate 52,500 pounds load when it

is refused at one half or more of the openings between protrusions.A “go” feeler gauge .002” thinner, or .037” thick, when it is acceptedat one half or more of the openings, will indicate that excessive loadhas not been applied. A test certificate relating gap to bolt load isprovided with each lot of DTI’s manufactred. Test certificates are alsoavailable relating customer specified load to gap.

Figure 3 shows the variation in bolt load (tension) on four 1 1/4” boltswhich were all torqed to 980 ft. lbs. without using Direct TensionIndicators. Four more 1 1/4” bolts from the same lot were tightenedusing DTI’s to a .039” gap. The variation in bolt load using DTI’s was farless. With uniform bolt load, your fastened joint is more reliable.

Broadly defined, torque is the force, or work, required to tension a bolThis is measured by calibrated wrenches. But the amount of work, ortorque, required to properly tension bolts is significantly affected by

the condition of the threads. As friction between nut and bolt threadsincreases, the amount of work required to install a bolt to a specifictension increases. The Direct Tension Indicators measure the resultingtension (bolt load). Input torque or procedures may change, but thebolt load indicated by the Direct Tension Indicators will be accurate.

Most installation problems in the field are caused by bolt conditionsCorrosion and dirt  – bolts that have been exposed to atmosphericconditions and weather in the field require extra work to tightento the specified tension because of corrosion and dirt that hasaccumulated in the bolt threads. Therefore the mechanic may need tuse a wrench of greater torque capacity.

Another way of overcoming the extra work caused by thread corrosioand dirt is by using clean, lubricated nuts and bolts. For example,

lubricants such as moly or nickel paste can reduce the coefficient offriction by as much as 50 percent.

Given a wrench of sufficient capacity, and fasteners that are cleanand lubricated, the time taken to properly install the bolt should begreatly reduced.

Calibrated Wrench TighteningMethod (980 lb. ft. torque)

Direct Tension IndicatorMethod

     B    o     l    t     T    e    n    s     i    o    n

      (     k     i    p    s     )

StationaryNut

HardenedSteel Washer

When the DTI is installed under the nut beingturned, a hardened steel washer must be

used betweeen the nut and the DTI.

DTI’s can be used with bevelwashers to accomodate over

a 1:20 bevel.

StationaryNut

BeveWash

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